hmm.. i think its eiji or mitsuki ... when eiji died you didnt see his face... the arm that he found could be someone elses and when mitsuki "dissapears" the guys hajime discribes got a white shirt right? so the only one with a shirt like that is mitsuki.. (she could hid hes boobs somehow..) whee... the next chapter is gonne be great!

Edit: Hajime was maybe drugged somehow and when he woke up mitsuki was gone... and why didnt he scream for help...?